In forma pauperis (IFP)
Permission to proceed in federal court without paying the filing fee.
In forma pauperis ("in the manner of a pauper") is a status that allows a litigant to proceed in federal court without paying the filing fee, because they cannot afford it.
The application is typically made on form AO-240, which requires sworn disclosure of income, assets, debts, and expenses. The standard is "unable to pay," not strict indigency. Pro se litigants file IFP applications most often; represented parties may also apply.